1. Field of the Invention
The invention relates to a high temperature pebble bed reactor having a reflector surrounding the core on all sides with means to remove the decay heat and with two different means for its shut-down. The first shut-down means comprises a plurality of absorber rods for insertion into the core for extended shut-down. The second shut-down means comprises a plurality of absorber rods displaceable in orifices in the lateral reflector for rapid shut-down or scram. The invention also provides for a process for the shut-down of the high temperature reactor.
2. Background of the Art
In general, high temperature reactors with spherical fuel elements are equipped with a shut-down and control system, which consists of a large number of core rods and/or reflector rods and whereby all operating and accidental events are safely controlled in the reactor. The choice of the group of absorbers to be used for the different shut-down functions (scram or long term shut-down), depends essentially on the capacity of the high temperature reactor involved.
Thus, in the case of the THTR-300MW.sub.el prototype, all shut-down processes are effected by means of the core rods while the reflector rods are intended for the reactor control processes. An example of such an arrangement is described in detail in U.S. Pat. No. 4,148,685, the disclosure of which is incorporated herein. In order to satisfy the prevailing high safety requirements, an emergency shut-down system is further available for use only if the core rods are not capable of operation (for example in the event of external accidents, such as the crash of an aircraft into the reactor structure). This emergency shut-down system comprises the introduction of a neutron absorbing gas or of small absorber pebbles into the core of the reactor.
A high temperature reactor for an intermediate capacity range (300 to 500 MW.sub.el), described in West German Offenlegungsschrift No. 3141734, is also equipped with a first shut-down system formed by the core rods and a second emergency shut-down system consisting of small absorber pebbles. The second shut-down system is used only in case of a failure of the first system.
A further high temperature reactor of the intermediate power range is described in West German patent application No. P3212322.1. Here again, a first shut-down system, consisting of absorber rods and a second shut-down system of small absorber pebbles to be used in case of the failure of the first shut-down system, are provided. In this reactor the first shut-down system comprises, in addition to the core rods, a number of reflector rods.
In high temperature reactors with a small power capacity (100 to 200 MW.sub.el), only reflector rods are required for the shut-down of the cores, which thus form the first shut-down system. Such a small high temperature reactor is described in West German application No. 3212264.0. The reflector rods, which are inserted from below into bore holes of the lateral reflector, perform both the scram and the long term shut-downs. A second shut-down system, here again consisting of small absorber pebbles, is actuated only upon a failure of the first system and, it effects a long term shut-down of the reactor.
A common feature of the aforedescribed high temperature reactors is that upon the failure of the first shut-down system a measure is actuated, which is afflicted by certain disadvantages. The feeding of absorbing agents into the reactor core, for example, leads to extended downtimes, due to the fact that the removal of the absorbing agent is very expensive. Furthermore, in addition to the absorber rods, supplemental absorber materials must be made available.
In another high temperature reactor known from West German Offenlegungsschrift No. 2451748, the introduction of absorber materials in the form of a gas or small pebbles is eliminated. This reactor possesses two different shut-down systems of which, one consists of core rods and effects long term shut-downs, while the other comprises a predetermined number of reflector rods whereby a scram is performed. In case of a failure of the reflector rods, a long term shut-down is actuated by means of the core rods. The shut-down reactivity of the reflector rods intended for a rapid shut-down is designed so that the reactor in all accident situations may be maintained subcritical for at least 30 minutes. This takes into consideration the fact that the reactor is being cooled to a greater or lesser extent by the principal cooling systems or the decay heat removing installations (in an extreme case down to the cold gas temperature); i.e. in relation to the negative temperature coefficient of high temperature reactors, there is overall a very high shut-down reactivity requirement. Furthermore, the reheating of the core after cooling involves a significant loss of time during the reactivation of the core to full power. Finally, the core installations are exposed to high thermal stresses during the change in temperature. Here again, the shut-down concept proposed is applicable only to high temperature reactors with a power range up to 300 MW.sub.el.